To read magnetic data recorded on a magnetic card and to record data onto the magnetic card, a card reader is widely used. As an application of the magnetic card, there is a cash card used for a transaction with a financial institution, for example. In recent years, a so-called skimming becomes a serious social problem where a criminal attaches a magnetic head to an insertion unit of the card reader to steal data recorded on the magnetic card. Although a countermeasure against the attachment of the magnetic head for skimming to the card insertion unit is underway, a criminal technique for skimming is also sophisticated, and there is a concern that the magnetic head for skimming is placed inside the card reader. Therefore, as a technique for detecting placement of a foreign matter such as the magnetic head inside the card reader, Patent Document 1 discloses arrangement of a capacitive sensor along a card passage inside the card reader. As a capacitive sensor to be arranged inside the card reader, a thin-type capacitive sensor is required, and thus, that which is configured so that a pair of comb-shaped electrodes or interdigital electrodes are placed on an insulating substrate, and further, its surface is covered with an insulating layer, is used. It is noted that as a capacitive sensor used for measurement of 4-terminal type, Patent Document 2 discloses a capacitive sensor where a pair of comb-shaped electrodes are arranged on an insulating substrate.